Polishing pads are useful in many applications. One such application is polishing glass work pieces. Regardless of the application, a polishing pad is moved relative to the object (e.g., glass, Si wafer, Sapphire wafer, etc.) being polished. This relative movement may be created by a rotating the polishing pad, by rotating the object being polished, or a combination of such movements. Other linear or any useful relative motion may be used between the polishing pad and the object being polished. In some embodiments, a force may be applied to press the polishing pad in contact with the wafer. The polishing may be performed to varying degrees such as to remove larger imperfections, to achieve a mirror finish and/or final flatness, etc.
Conventionally, the process of polishing glass work pieces to remove lapping imperfections is accomplished by a mechanochemical process in which one or more polishing pads, typically made of urethane, is used with a polishing solution (slurry), commonly comprising fine abrasive particles such as cerium oxide or zirconium oxide. The glass work piece is supported between a platen covered with a polishing pad and a carrier to which the work piece is attached, or, in the case of double-sided polishing, the work piece is held between two platens, each covered with a polishing pad. The pads are typically about 1 mm thick and pressure is applied to the wafer surface. The planar work piece is mechanochemically polished by relative movement between the platen and the work piece. Variations on such polishing systems may also be used to polish a planar work piece.
During polishing, pressure is applied to the planar work piece surfaces by pressing the pad and the work piece together in a polishing tool, whereby a uniform pressure is generated over the entire planar surface owing to the compressive deformation of pads. Polishing tools often have dynamic heads which can be rotated at different rates and at varying axes of rotation. This action removes mass from the work piece and thus the damage from the work piece lapping process.
The polishing pad can, for example, be a polyurethane polishing pad. Typical polyurethane polishing pads are designed to be attached to a planar platen for polishing a planar work piece. However, increasingly, specialty glass work pieces are being used that have non-planar surfaces or that have surfaces comprising non-planar portions. Unfortunately, standard polyurethane polishing pads cannot effectively polish a non-planar work piece. One of the reasons that typical polishing pads are unacceptable for non-planar work pieces is that they do not provide even polishing over the whole surface. Standard polishing pads are generally unable to maintain contact with the entire surface area of the non-planar work piece. Therefore, over certain portions of the work piece's surface, they are unable to remove scratches (relics of the work pieces' lapping process) or other imperfections, in the normal course of polishing a non-planar surface with a planar polishing pad, the polishing pad will not be able to compress enough to effectively contact the entire surface area of the work piece. The use of compressible planar polyurethane foam or a stacked, planar foam composite may help provide additional coverage but still does not contact all surface areas in cases of significant curvature. Work pieces that are not completely polished may wind-up being scrapped as rejected parts.
To counteract these problems, one solution is to increase the force pressing the polishing pad to the non-planar surface to be polished. The increased force is intended to force the polishing pad to conform to the non-planar surface. However, these efforts have proven ineffectual. Either the force is still not enough to uniformly polish the entire surface of the glass to be polished, or the pressure is excessive and causes breakage of the glass. In another attempted solution, typical polishing pads that are planar in nature and of varying degrees of compressibility cannot effectively contact the entire surface area of the work piece to be polished. Also, typical polishing pads that are softer and/or more compressible are of a design that wears out faster, are slow to polish compared to stiffer pads (e.g., a planar surface polyurethane polishing pad), and/or cause defects.
Thus, the industry has turned away from polishing pads for polishing non-planar surfaces, and has widely implemented the use of brushes, flaps, and the like. The brushes or flaps are designed to contact all the surfaces. The flaps may be made of a felt material, a carpet like material, and or the like. The brushes or flaps are designed to polish the non-planar portion of the glass work piece. For example, a rectangular piece of glass may be turned vertically to present a first edge to be polished. The brushes or the flaps may rub over the first edge, to polish that non-planar area. Then the glass can be rotated in its carrier to present a second edge for similar treatment. This process is then repeated for each edge. Although cumbersome, time consuming, expensive, and increasing of the likelihood of damaging the glass, this is the currently preferred process.
One such example process is described now in further detail. A cover glass having both planar and non-planar surfaces is polished using brushes. The cover glass in this example has four sides. The cover glass “blanks” are loaded into “boats” with about 120 pieces to be polished at a time using a customized polishing machine. The cover glass blanks are each oriented in a vertical orientation presenting one of the four sides to the brushes. The non-planar portion of the cover glass blank, near the edge on that side, is polished by the brushes. This polishing takes about 10 to 15 minutes to polish that side of the cover glass for all 120 pieces. Next, the cover glass blanks are rotated 90 degrees to polish a non-planar portion near the edge on a second side of the cover glass. It may take about five minutes to rotate all 120 pieces. After polishing for the second side, the cover glass blanks are rotated again. A non-planar portion near the edge of the third side of the glass is then polished. The cover glass blanks are rotated again, and a non-planar portion near the edge of the fourth side is polished. In one example, the polishing of the non-planar portions of the four sides of the glass takes a total elapsed time of 55 to 75 minutes to complete. The brush life is about four days.
Once all of the non-planar surfaces near the edges of the work piece are polished, then the glass can be placed in a standard 9B polishing machine, in a horizontal carrier, to polish the cover glass' planar portion(s). In one such machine, 12-15 cover glass pieces can be polished at one time. This step may take about 25 minutes for each set of 12-15 pieces.
Some experimentation has also been done in using woolen material and carpet bits as flaps, as an alternative to the brushes. These solutions are similar in costs to the brushes, but are more efficient in polishing (e.g., in 8-10 minutes). These materials have a life of about 6-8 days. Nevertheless, these materials would still involve the use of the five step process described above.
Thus, the use of brushes or flaps to polish non-planar work pieces can be very time consuming. Moreover, use of brushes or flaps can involve five separate polishing steps with intervening set up steps. Each intervening handling step increases the risk of breaking the glass pieces and adds significant amounts of time to the overall process. Moreover the life of the materials is relatively short, so there is an increase in cost and time associated with changing out the polishing medium. For example, the brushes of flaps may need to be replaced every 24 hours due to brushes or flaps becoming disengaged or fatigue that caused the pad to be ineffectual. The brushes or flaps may be “composite pads” for example, polyurethane impregnated felt or synthetic felt pads. Such brushes or flaps are sometimes used, but they can cause surface damage, have a short life, and are expensive compared to polyurethane pads.
Therefore, a need exists for improved systems, methods, and devices for polishing work pieces having surfaces that have non-planar portions.